China is one of the largest citrus producing countries, and the citrus industry is one of the mainstay industries of agriculture in China, which occupies a very important place in the agricultural economy and plays a very crucial role in increasing farmers' income, increasing social employment and building a new socialist countryside. Diseases and insect pests of citrus are great obstacles to the development of the citrus industry, wherein the most serious one is the citrus huanglongbing which is known as a “cancer” of the citrus. With fast development of the citrus industry, the occurrence and damage of citrus huanglongbing become increasingly serious. This disease is an infectious plant disease with great harm, with a long lasting period of epidemic, a wide incidence range and a high incidence rate, which leads to a short lifetime, a low yield and a high production cost of citrus in most areas of China, and leads to an annual economic loss of up to billions of dollars, seriously impeding and restricting the development of the citrus industry in China and even around the world. Research results have proven that the huanglongbing pathogen in the field is mainly transmitted through Diaphorina citri. Therefore, for controlling the huanglongbing, it is the core issue to control Diaphorina citri and reduce the population quantity of Diaphorina citri in the field. Guangdong Entomological Institute had made a relatively thorough research in respect of the Diaphorina citri and the citrus huanglongbing, and has applied for an invention patent titled “Method for Controlling Diaphorina Citri and Citrus Huanglongbing in Citrus Orchards” with the application number of 201310049098.6.
However, the main problem for the control of the citrus huanglongbing at present is high difficulties, high technical requirements, and difficulties in implementation. The key of the problem lies in that since the detection and diagnosis technique cannot keep up, the farmers do not consider that their citrus trees have been infected with the citrus huanglongbing, and thus they take no active measures. Therefore, for the control of citrus huanglongbing, it is a technical difficulty in urgent need to develop a detection and diagnosis technique which is fast-ready and easily operated.
Currently, the detection and diagnosis technique mainly includes diagnosis of field symptoms and identification via an indicator plant, diagnosis through electron microscope detection, diagnosis through immunological detection, diagnosis through detection of nucleic acid molecules and a diagnosis method based on the starch iodine reaction (Zhang Liping, 2009), wherein the PCR technique is a mainstream technique at present (Deng Xiaoling et al., 1999; Wang Zhongkang et al., 2004; Meng Xiangchun et al., 2007). The diagnosis method based on the starch-iodine reaction was derived as follows. Schneider (1968) found that a leaf infected with the citrus huanglongbing has very high starch content, and in 2002, by using a histological method, Masatoshi Onuki proved that a large number of starch grains are accumulated in a citrus leaf infected with the citrus huanglongbing. Le Thi Thu Hong and Nguyen Thi Ngoc Truc (2003) subsequently built the technique which diagnoses the citrus huanglongbing through the iodine reaction, with the following key points: (1) collecting leaves with the features of the citrus huanglongbing in the morning, collecting no inner leaves, rolled leaves, tender leaves, branches, fruits and roots; (2) taking 1 g of leaves, mixing it with 2 ml of distilled water and grinding; (3) dropwise adding 2 μL of mixed liquor of leaves and water onto a reaction film (NCM, NitroCellulose Membrane), and after 5 minutes dropwise adding 2 μL of iodine solution onto the sample on the reaction film; and (4) observing the color change of the leaves to obtain a result, wherein if a negative reaction occurs, the color is not changed, and if a positive reaction occurs, the color is changed to blue. In Japan, TAKUSHI (2007) measured that the content of starch in the citrus leaves infected with the citrus huanglongbing is 400-500 mg/kg while the content of starch in normal leaves is 85.6 mg/kg, and built a rapid diagnosis technique. In the technique, they scratched a surface of a leaf with an abrasive paper for 20 times, subsequently put the abrasive paper into a plastic bag, and added 25 μL of iodine solution into the bag for color development. In Malaysia, Lily Eng (2007) repeated the above method by using different abrasive paper. In China, Zhang Liping et al. (2009) repeated the technique by the following steps: grinding the collected citrus material which were infected with the disease; adding an equivalent amount of pure water; then drawing 4 μL of mixed liquor of leaves and water onto the reaction film; and after 5 min, adding an equivalent amount of iodine solution and observing the color change, wherein the consistency between the experiment result and the PCR result reached 93.3%. Through researches, the inventor found that the reasons for the unwide application of starch color development technique are as follows: (1) since the sample is captured in the morning, the interference of starch in normal leaves are not truly eliminated, while sometimes due to environmental factors, starch accumulated by photosynthesis of normal leaves may not be transferred away in the evening and thus the starch remains in normal leaves in the morning; (2) since the chlorophyll is not removed by the aforementioned technique, interference of the chlorophyll exists during the color development and may affect the accuracy of detection and diagnosis; and (3) in the aforementioned diagnosis technique, the leaves need to be mixed with water for grinding, but the starch is insoluble in water, such that the accuracy of the color development reaction when the leaves are mixed with water is affected.